Abstract: Hot-dip aluminizing and diffusion annealing were carried out on 25MnV steel round link chains for mining.The microstructure, microhardness, stretching properties and breaking properties of the coatings were studied.The results show that the coatings consist of aluminum layer and Fe-Al alloy layer.The major phase of the alloy layer is Fe₂Al₅ and FeAl₃, and the area of high hardness of the alloy is larger.After diffusion annealing treatment at 850 ℃for 4 h, the aluminum layer transforms into alloy layer completely.The alloy layer can be divided into an outer diffusion layer and inner diffusion layer.The major of the outer diffusion layer is FeAl and the inner diffusion layer is mainly composed of Fe₃Al, and the region of high hardness in the alloy layer is smaller.The microhardness of the inner diffusion layer is lower than the substrate.After diffusion, due to the Fe-Al alloy layer with high temperature oxidation resistance, the oxidation of tension piece in the process of quenching and tempering reduces and then the tensile strength and yield strength improve.The elongation and shrinkage of the diffused aluminized tensile parts reduce result from high hardness of Fe-Al alloy layer.Compared with the non-treatment chain, the breaking load of the aluminized chain and the diffused aluminized chain show little change, but the elongation both obviously decreases.In addition, their properties both meet the Chinese standard of C-level ring chain.